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unrrsn sra'ras PATENT OFFICE.

HUGH K. MOORE, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR '10 BROWN COMPANY, OF

' BERLIN, NEW HAMPSHIRE, A CORPORATION MAINE.

METHOD OF REMOVING CEB'IAIH IMPTJ'BITIES IEROIVJ'. ELECTROLYTIC CELLS.

No Drawing.

Y fication.

In the production of chlorin and caustic soda, it is now common practice to electrolyze solutions of sodium chlorid and water in an electrolytic cell in which the anode and the cathode are separated by a porous diaphragm consisting usually of one or more layers of asbestos paper or cloth; and, according to approved practice, the cathode chamber is provided with an outlet to conduct from it the solution of caustic soda and electrolyte which flows to the bottom of the chamber, as shown, for example,

in Letters Patent No. 1,073,559, dated September 16, 1913. Common salt, such as used in electrolytic cells, contains many impurities, such for example as salts of magnesium and calcium, and, even after the brine has been subjected to the usual and commercial refining and purifying processes, it still contains certain quantities of such magnesium and calcium salts. When pure sodium chlorid solutions are employed as the electrolyte, no gelatinous products of decom position are formed on the diaphragm, but, when the salts of magnesium or calcium or both are present in the electrolyte, the hydroxids of these metals and the basic salts of such metals are deposited on the diaphragm, suchafor example as calcium hydroxid, CaO IrL, basic chlorid of calcium,

of the cell, so that the electrical power per unit of caustic and chlorin is greatly in- Specification of Letters Patent.

in diiferent ways.

Patented duty 8, 1919.

Application filed March 25, 1919. Serial No. 284,980.

creased. Not only do these gelatinous deposits form in the diaphragm, but they alsc form in the pores or interstices of the cathode, in which case the hydrogen may not freely escape from the cathode but finds its way into the anode compartment of the cell, thereby forming, with chlorin, an explosivemixture which may be detonated by sunlight or an electric spark, with accompanying danger to the operatives.

Herctofore, when the gelatinous deposit has reached a point where the eiiiciency of the cell was lowered, the only available remedy has been stripping the cells andreplacing the clogged diaphragms with fresh sheets of asbestos. This results deleteriously In the first place, the stripping of the cell and the replacement of new diaphragms is expensive, the cost of the diaphragm being a considerable item of expense, in addition to that of the labor involved. Moreover, it is practically impossible to ascertain in advance Whether the dia phragm is perfect, and practical experience has shown that diaphragms are frequently imperfect, so that, after the cell has been equipped with a new diaphragm, it is necessary to stripit again for the replacement of a fresh diaphragm. In the second place, a fresh or new diaphragm is highly porous, and, until it has been used for a time, permits the percolation of an excess quantity of the electrolyte, with a diminished decomposition eiiiciency. As a result, the effluent of the cell contains an undue quantity of salt, which, when precipitated, has an abrasive and deleterious action on the pipes and tubes of the evaporating and concentrating apparatus used in recovering the caustic soda; and moreover the presence of an excess quantity of salt holds by entrainment an excess quantity of caustic soda, when the salt is separated in the evaporating and concentrating process. Since the salt is again formed into brine for re-use in the cells, and cannot be used in itsalkaline condition, the caustic must be neutralized with hydrochloric acid, thereby addin to the cost of operation. Hence, from the oregoing, it will be apparent that an old or used diaphragm, provided it is not clogged sufficiently to prevent the flow required to attain the desired current and decomposition efiiciency, is better than a new diaphragm.

I have discovered that it is possible, by the use of acids which will not affect the diaphragm or materially atlect the cathode and the cell body, to decompose the gelatinous deposits and form soluble salts which will pass through the diaphragm, so that the diaphragm will be substantially freed therefrom. No acid can be used which by its re action with the deposit will term an insoluble precipitate. Lactic acid (CH CH(0H) CODE) 1 have found will meet the requirements.

In practice the cell is by-passed electrically and the electrolyte is withdrawn therefrom. Then the anode compartment of the cell is tilled with a weal: aqueous solution of lactic acid of such strength that the cell will be freed from its impurities in convenient working hours. T hat is to say, the strength of the solution depends upon the time which the operator may desire to have the cell out of use. For example, an .8 solution will require approximately only half the time required for a .-l% solution. The lactic acid solution is fed continuously to the cell, the usual float. valve being employed to regulate the flow and keep the required level of liquid in the cell, and, since the, solution (after passing through the diaphragm) flows down the diaphragm and is removed from the cell, the continued flow washes the soluble reaction product from the diaphragm.

After this operation has been carried on long enough to free the cell of said impurities to the desired extent, the remaining lactic acid solution in the cell is withdrawn, and the cell is filled in the usual manner with brine and the electrical current is turned on, so that the cell thereafter functions in high state of efiiciency.

While lactic acid theoretically attacks iron, yet in the relat-h 'ely weak solutions used I find that the reaction on the iron is so slight as to *be wholly negligible.

The products of the reaction of the lactic acid upon the deposits on the diaphragm are lactates of calcium and magnesium which are soluble in water. They pass, with the solution, through the diaphragm and are washed from the cathode-side thereof and are withdrawn with the solution from the cathode compartment.

In the case of a cell in which the cathode is normally submerged, the cathode com partment is first washed before the anode chamber is filled with the acid solution. Any other acid (preferably organic), which will produce the desired results without materially deleterious effect, may be used in lieu of lactic acid.

What I claim is 1. The herein described process of freeing electrolytic cells from gelatinous deposits of calcium and magnesium or either, which consists in decomposing the same with an acid which produces a soluble reaction product.

2. The herein described process of freeing the diaphragm of an electrolytic cell from deposits of calcium or magnesium or both, which consists in passing through such diaphragm an acid solution which reacts with such deposit to form a soluble reaction product.

3. The herein described process of freeing the diaphragm of an electrolytic cell from deposits of calcium or magnesium or both,

which consists in passing through such diaphragm an acid solution which reacts with such deposit to form a soluble reaction prodact, and by a continued flow of Such solu tion washing such reaction product from the diaphragm.

4, The herein described process of freeing electrolytic cells from deposits of calcium or magnesium or both, which consists in decomposing the same with an organic acid solution capable of producing soluble reaction roducts, and removing such products from t e cell.

5. The herein described process of freeing electrolytic cells from deposits of calcium or magnesium or both, which consists in decomposing the same with a lactic acid solution thereby forming soluble lactates.

6. The herein described process of freeing electrolytic cells from deposits of calcium or magnesium or both, which consists in passing through the diaphragm a solution of lactic acid, forming soluble lactates, and washing such lactates from the diaphragm.

In testimony whereof I have aflixed my signature.

HUGH K. Moose. 

